1. Field of the Invention
The present invention relates to a biological material detection element, biological material detection method and apparatus, and charged material moving apparatus which are used to detect biological materials such as genes and proteins.
2. Description of the Related Art
Recently, systems for detecting biological materials such as genes and proteins have been under development. For example, the detection of genes is used for the prediction of the curative effect produced by interferon. A conventional biological material detection technique will be described with reference to an example of the prediction of curative effect produced by interferon.
It is known that when a person is infected with hepatitis C, this disease is likely to proceed to hepatic cancer through hepatic hepatocirrhosis. One of the medical treatments for the disease is a method using interferon. It is reported that the injection of interferon has a curative effect on only about 20 to 30% of the Japanese, and causes strong side effects even if it produces the curative effect. For this reason, attention has recently begun to be paid to personalized medical treatment in which the curative effect of interferon is predicted, and interferon is used only when a curative effect can be expected.
As a method of predicting the curative effect of interferon, a method of checking the type of virus and the amount of virus at the gene level is known. It is thought that interferon has little effect on type 1b with which many Japanese are infected, but does have a curative effect on type 2a, and that interferon has little effect when the amount of virus is 106 copy/mL or more. In actual diagnoses, these factors are often mixed, resulting in difficulty in prediction. Recently, as a method of predicting the curative effect of interferon, a method has been reported, in which single nucleotide polymorphism (SNP) that exists in the promoter region of gene that codes for MxA protein is used as a marker. According to this report, if the SNP is of type G/G, the effect of interferon is small, whereas if the SNP is of type G/T or T/T, interferon works effectively.
As described above, it is becoming possible to predict the curative effect of interferon by analysis at the gene level. All these methods have used cumbersome, expensive conventional techniques (electrophoresis, Microplate EIA, and the like), and hence more simplified methods have been required for clinical examination.
Under the circumstances, attention has recently been paid to a gene inspection technique using a biological material detection element called a DNA chip (Beattie et al. 1993, Fodor et al. 1991, Khrapko et al. 1989, and Southern et al. 1994). The DNA chip is formed from a several cm square glass or silicon chip on which a plurality of types of DNA probes with different sequences are immobilized. A mixture of a sample gene marked by a fluorescent dye, radiation isotopic element (RI), or the like or non-marked sample gene and marked oligonucleotide is caused to react on the chip. If there is a sequence, in the sample, which is complementary to a DNA probe on the chip, a signal originating from the marker can be obtained at a specific portion on the chip. If the sequences and position of the immobilized DNA probes are known in advance, a base sequence existing in the sample gene can be easily checked. Such a DNA chip makes it possible to obtain many kinds of information concerning base sequences by one test, and hence can be used for a clinical diagnosis technique (Pease et al. 1994, Parinov et al. 1996).
The principle of an electrochemical gene detection method using a DNA chip is schematically shown in FIG. 25.
In a typical conventional DNA chip, a sample liquid made of an aqueous solution of genes is introduced from a sample liquid introduction portion placed on one end portion of the chip surface, flows on various DNA probes immobilized in the cells of a matrix, and then is discharged from a sample liquid discharge portion placed on the other end portion of the chip surface. The overall DNA chip is covered with a resin case, and the portion where the DNA probes are immobilized is made transparent to read optical signals such as fluorescence.
The above conventional DNA chip is designed such that in the process in which a sample liquid flows from one end portion of the chip surface to the other end portion, the liquid is guided on the DNA probes arrayed in the form of a matrix. Since it is difficult to make the sample liquid uniformly flow on the DNA probes, it is difficult to make the gene and a DNA probe reliably react with each other. This tends to cause variations in detection result.
In addition, in general, since the gene concentration in a sample liquid is low, when a conventional DNA chip has no gene concentrating effect, in particular, a gene to be detected must be amplified in advance by a gene amplification method such as the PCR method.
Conventionally, in a biological material detection apparatus for detecting genes by electrochemical measurement using a DNA chip, current measurement is performed while a voltage is applied between electrodes in a proper electrolytic solution stored in a vessel 100, as shown in FIG. 27. Electrodes 101, 102, and 103 called a counterelectrode, reference electrode, and working electrode, are inserted in the vessel 100.
The reference electrode 102 is an electrode for applying a reference potential to the counterelectrode 101, which is held at a predetermined potential. A voltmeter 106 is connected between the counterelectrode 101 and the reference electrode 102 to measure the potential of the counterelectrode 101. Besides, a variable DC voltage source 104 is connected between the counterelectrode 101 and the working electrode 103. The variable DC voltage source 104 varies an applied voltage between the counterelectrode 101 and the working electrode 103. The voltage sweeping causes a current change, which is measured by an ammeter 105, thus detecting a gene.
FIG. 28 shows a procedure for detecting a gene by biological material detection apparatus having an arrangement like that shown in FIG. 27 using a nucleic acid intercalating agent. First of all, a sample liquid is supplied (step S1). With this operation, the sample liquid is made to adhere to a DNA probe (single stranded DNA that reacts with a specific gene) immobilized to the working electrode to convert the DNA in the liquid into a single stranded DNA, thus performing hybridization. The sample liquid that did not adhere the DNA probe is then cleaned (step S2). Subsequently, an intercalating reagent (nucleic acid intercalating agent) that reacts specifically with a double stranded DNA is supplied to improve the detection sensitivity (step S3), and the unnecessary intercalating agent is further cleaned (step S4). Finally, a voltage is applied between the counterelectrode 101 and the working electrode 103, and an oxidation current obtained from the intercalating agent is measured, i.e., an electrochemical signal obtained from the intercalating agent is measured (step S5).
A current-potential curve of Hoechst 33258 as a DNA binder is shown in FIG. 26.
As described above, in the conventional biological material detection apparatus, since current measurement can be performed only once with respect to one working electrode (DNA chip), it is difficult to improve the gene detection sensitivity. FIG. 29 shows a change in current density when a given plasmid (pYRB259) is measured by using an electrochemical DNA chip. As is obvious, since the background current is high, a low-concentration gene cannot be detected. In general, since the gene concentration of a sample liquid is low, a target gene must be amplified in advance by a gene amplification method such as the PCR method.